Photoresist compositions comprising either a photosensitive polyimide or a polyamic acid as a precursor therefor have conventionally been used. These compositions in practical use are mostly of the negative type used in the following manner. The composition is applied to a substrate, and the coating is irradiated with actinic rays through a given photomask to thereby reduce the solubility of the exposed areas in a developing solution. The unexposed areas are then removed by dissolving the same in the developing solution to thereby form the desired negative pattern on the substrate.
These photoresist compositions comprising a photosensitive polyimide or a precursor therefor, which after irradiation with actinic rays leave the exposed areas on the substrate, have an advantage that the pattern obtained has excellent heat resistance. Because of this and other advantages, various proposals thereon have been made so far. Examples thereof include a technique of incorporating methacryloyl groups into a polyimide precursor through ester bonds or ionic bonds (see, for example, JP-A-49-11541 (the term "JP-A" as used herein means an "unexamined published Japanese patent application"), JP-A-50-40922, JP-A-54-145794, and JP-A-59-38038) and a self-sensitizing polyimide having a benzophenone skeleton and having alkyl groups in the nitrogen-bonded ortho positions (see, for example, JP-A-59-219330 and JP-A-59-231533).
However, such conventional negative photoresist compositions comprising a photosensitive polyimide or precursor therefor have problems, for example, that the photoresist film swells during development and that the unexposed areas of the photoresist film partly remain undissolved due to light diffraction during exposure through a photomask. The conventional photoresist compositions have another problem that the photoresist film has a degree of shrinkage as high as about 50% at heat curing and the pattern obtained has insufficient dimensional stability. Furthermore, the conventional compositions necessitate use of a developing solution containing an organic solvent and hence pose problems concerning working atmosphere, waste liquor treatment, etc.
Still a further problem of the latter photoresist composition described above is that since the polymer skeleton structure is limited, the properties of the film finally obtained are limited. Namely, the prior art composition failed to flexibly meet a variety of property requirements.
Under these circumstances, high-sensitivity negative photoresist compositions which can be obtained by incorporating as a photosensitizer a 1,4-dihydropyridine derivative capable of forming a salt having a pyridine skeleton upon irradiation with actinic rays into a polyamic acid as a polyimide precursor and are effective in eliminating the above-described various problems of conventional negative photoresist compositions comprising a photosensitive polyamic acid are proposed in JP-A-6-75376, JP-A-7-271034, and JP-A-10-39510.
However, these negative photoresist compositions still have problems that a developing solution penetrates into the interface between the photoresist film and the substrate during development to cause lifting at peripheral parts of the exposed areas, and that the polyimide film finally obtained through heat curing shows insufficient adhesion to the substrate.